Headlight devices for vehicles need to have a predetermined light distribution pattern specified by road traffic rules or the like. “Light distribution” refers to a luminous intensity distribution of a light source with respect to space. That is, it refers to a spatial distribution of light emitted from a light source. Further, “luminous intensity” indicates the degree of intensity of light emitted by a luminous body and is obtained by dividing the luminous flux passing through a small solid angle in a given direction by the small solid angle.
As one of the road traffic rules, for example, a predetermined light distribution pattern for an automobile low beam has a horizontally long shape narrow in an up-down direction. To prevent an oncoming vehicle from being dazzled, a boundary (cutoff line) of light on the upper side of the light distribution pattern is required to be sharp. That is, a sharp cutoff line with a dark area above the cutoff line (outside the light distribution pattern) and a bright area below the cutoff line (inside the light distribution pattern) is required.
“Cutoff line” here refers to a light/dark borderline formed when a wall or screen is irradiated with light from a headlight, and a borderline on the upper side of the light distribution pattern. That is, it refers to a light/dark borderline on the upper side of the light distribution pattern. It refers to a borderline on the upper side of the light distribution pattern and between a bright area (inside of the light distribution pattern) and a dark area (outside of the light distribution pattern). Cutoff line is a term used when an irradiating direction of a headlight for passing each other is adjusted. The headlight for passing each other is also referred to as a low beam.
The illuminance is required to be highest at a region on the lower side of the cutoff line (inside the light distribution pattern). The region of highest illuminance is referred to as the “high illuminance region.” Here, “region on the lower side of the cutoff line” refers to an upper part of the light distribution pattern, and corresponds to a part for irradiating a distant area, in a headlight device. To achieve such a sharp cutoff line, large chromatic aberration, blur, or the like must not occur on the cutoff line. “Blur occurs on the cutoff line” indicates that the cutoff line is unclear.
Further, as another example of the road traffic rules, for identification of pedestrians and signs, it needs to have a “rising line” along which the irradiation on a walkway side rises. This is in order to visually recognize people, signs, or the like on the walkway side without dazzling oncoming vehicles. “Rising line along which the irradiation rises” here refers to the shape of the light distribution pattern of a low beam that is horizontal on an oncoming vehicle side and obliquely rises from the oncoming vehicle side toward a walkway side.
The “low beam” is a downward beam and used in passing an oncoming vehicle or the like. Typically, the low beam illuminates an area about 40 m ahead. Further, “up-down direction” refers to a direction perpendicular to the ground surface (road surface). A vehicle headlight device needs to provide this complicated light distribution pattern.
To provide such a complicated light distribution pattern, a configuration using a light blocking plate or the like is commonly used. In this configuration, the light blocking plate or the like blocks light, thereby reducing use efficiency of light. Hereinafter, use efficiency of light will be referred to as “light use efficiency.”
Patent Reference 1 discloses a technique that forms a cutoff line by using a light blocking plate.